To improve an integration of circuit cells, a three-dimensional integrated circuit (3D IC) is developed. The 3D IC includes two or more layers of active electronic components integrated vertically and horizontally into a single circuit. In some applications, the 3D IC is manufactured with fabrication process in two or more stacked tiers. For example, active devices in an upper tier are formed on a semiconductor layer over the interconnect structure in an adjacent lower tier, in which the upper tier is stacked above the adjacent lower tier.